function loadImg (url) {
  return new Promise((resolve, reject) => {
    const img = new Image()
    img.onload = function () {
      console.log('一张图片加载完成');
      resolve(img)
    }
    img.onerror = function () {
      reject(new Error('cound not load image at' + url))
    }
    img.src = url
  })
}
